Optical fiber

ABSTRACT

The present invention relates to an optical fiber having, at least, a structure for effectively restraining microbend loss from increasing. This optical fiber is an optical fiber suitable for a dispersion-flattened fiber, a dispersion-compensating fiber, and the like, and insured its single mode in a wavelength band in use. In particular, since the fiber diameter is 140 μm or more, this optical fiber has a high rigidity, so that the increase in microbend loss is effectively suppressed, whereas the probability of the optical fiber breaking due to bending stresses is practically unproblematic since the fiber diameter is 200 μm or less. Also, since this optical fiber has a larger mode field diameter, it lowers the optical energy density per unit cross-sectional area, thereby effectively restraining nonlinear optical phenomena from occurring.

RELATED APPLICATIONS

[0001] This is a Continuation-In-Part application of application Ser.No. PCT/JP99/03672 filed on Jul. 7, 1999, now pending.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an optical fiber which issuitable as an optical transmission line in wavelength divisionmultiplexing (WDM) transmission systems.

[0004] 2. Related Background Art

[0005] WDM transmission lines which enable optical transmissions, thoseof a large capacity and high speed in particular, mainly utilize opticalfibers. Recently, however, the deterioration in light signals caused bynonlinear optical phenomena such as four-wave mixing among individuallight signals occurring in an optical fiber has become problematic insuch WDM transmission systems. Therefore, in the WDM transmissionsystems, it is important that the occurrence of nonlinear opticalphenomena be suppressed, and to this aim, it is necessary that the modefield diameter or effective area of the optical fiber be increased, soas to lower the optical energy density per unit cross-sectional area.For example, Japanese Patent Application Laid-Open No. HEI 8-248251discloses an optical fiber having an effective area (70 μm² or more)which is greater than that of normal dispersion-shifted fibers.

SUMMARY OF THE INVENTION

[0006] It has been known that, in general, microbend characteristicsdeteriorate as the mode field diameter or the effective area increases,whereby the microbend loss caused by cabling becomes greater.

[0007] For example, FIG. 1 is a chart showing the refractive indexprofile of a dispersion-shifted fiber having a double core structure. Inthis dispersion-shifted fiber, the core region is constituted by aninner core having a refractive index n1 and an outer core having arefractive index n2 (<n1), whereas a single cladding layer having arefractive index n3 (<n2) is provided on the outer periphery of the coreregion. On the other hand, FIG. 2 is a graph showing the relationshipbetween the mode field diameter and the increase in loss caused bymicrobend at a wavelength of 1.55 μm (1550 nm) concerning this opticalfiber having the refractive index profile of a double core structure. Inthis specification, the mode field diameter refers to Petermann-I modefield diameter. This Petermann-I mode field diameter is given by thefollowing expressions (1a) and (1b):

MFD1=2·w ₁  (1a) $\begin{matrix}{w_{1}^{2} = {2 \cdot \frac{\int_{0}^{\infty}{{E^{2} \cdot r^{3}}\quad {r}}}{\int_{0}^{\infty}{{E \cdot r}{r}}}}} & \text{(1b)}\end{matrix}$

[0008] as shown in E. G. Neumann, “Single-Mode Fibers,” pp. 225, 1988.

[0009] In expression (1b), r is the radial positional variable from thecore center taken as the origin, whereas E is the electric fieldamplitude and is a function of the positional variable r. The microbendloss is defined by the amount of increase in loss when an optical fiberhaving a length of 250 m is wound at a tension of 100 g around a bobbinhaving a barrel diameter of 280 mm whose surface is wrapped with a JIS#1000 sandpaper sheet.

[0010] Also, from the results of theoretical studies, it has been knownthat the relationships of the following expressions (2a) to (2c) existbetween microbend loss Δ α and mode field diameter MFD1: $\begin{matrix}{{\Delta\alpha} = {\frac{1}{4} \cdot \left( \frac{1}{R^{2}} \right) \cdot \left( {k \cdot {n1} \cdot w_{1}} \right)^{2} \cdot {\Phi ({\Delta\beta})}}} & \text{(2a)} \\{{\Delta\beta} = \frac{1}{w_{1}^{2} \cdot k \cdot n_{1}}} & \text{(2b)} \\{{\Phi ({\Delta\beta})} = {\pi^{\frac{1}{2}} \cdot {Lc} \cdot {\exp \left\lbrack {- \left( \frac{{\Delta\beta} \cdot {Lc}}{2} \right)^{2}} \right\rbrack}}} & \text{(2c)}\end{matrix}$

[0011] In these expressions, R is the radius of curvature ofmicrobending applied to the optical fiber, k is the wave number, n1 isthe refractive index of the core region, and Lc is the correlationlength of the microbending applied to the optical fiber.

[0012] As can be seen from FIG. 2 and expressions (2a) to (2c) mentionedabove, the microbend loss increases as the mode field diameter MFD1 isgreater. However, though the conventional optical fibers are designed inview of macrobend loss, no consideration has been given to microbendloss. Also, it has been known that, if the amount of increase in lossmeasured when an optical fiber is wound around a bobbin whose surface iswrapped with sandpaper, as an index for cabling an optical fiber,exceeds about 1 dB/km, then microbend loss increases upon cabling.Hence, it is clear that microbend loss increases upon cabling in anoptical fiber such as the one mentioned above.

[0013] In order to overcome such problems, it is an object of thepresent invention to provide an optical fiber having, at least, astructure which can effectively suppress the increase in microbend loss.

[0014] For achieving the above-mentioned object, the optical fiberaccording to the present invention comprises a core region extendingalong a predetermined axis and a cladding region provided on the outerperiphery of the core region, these core and cladding regions beingconstituted by at least three layers of glass regions having respectiverefractive indices different from each other. Also, this optical fiberis substantially insured its single mode with respect to light at awavelength in use, e.g., in a 1.55 μm wavelength band (1500 nm to 1600nm), and has a fiber diameter of 140 μm or more but 200 μm or less.Thus, since the fiber diameter is 140 μm or more, the rigidity of theoptical fiber according to the present invention is high even when themode field diameter is large, whereby the increase in microbend loss issuppressed. On the other hand, since the fiber diameter is not greaterthan 200 μm, the probability of the optical fiber breaking due tobending stresses is practically unproblematic.

[0015] In particular, when the 1.55-μm wavelength band is employed asthe wavelength band in use for WDM transmissions, it is preferred in theoptical fiber according to the present invention that the absolute valueof chromatic dispersion at a wavelength of 1550 nm be 5 ps/nm/km orless. Also, it is preferred that the Petermann-I mode field diameter be11 μm or more. It is because of the fact that, if the mode fielddiameter is 11 μm or more, then the optical energy density per unitcross-sectional becomes smaller even when WDM signals are transmitted,whereby the occurrence of nonlinear optical phenomena can effectively besuppressed.

[0016] The optical fiber according to the present invention can beemployed as a single-mode optical fiber such as dispersion-shiftedfiber, dispersion-flattened fiber, dispersion-compensating fiber, or thelike.

[0017] In particular, when the optical fiber according to the presentinvention is employed as a dispersion-flattened fiber, it is preferablefor the optical fiber to have, for at least one wavelength within thewavelength band in use, a dispersion slope of 0.02 ps/nm²/km or less andan effective area of 50 μm² or more. More preferably, inparticular, thedispersion slope is 0.02 ps/nm²/km or less in terms of absolute value.

[0018] Also, when the optical fiber according to the present inventionis employed as a dispersion-compensating fiber, it is preferable for theoptical fiber to have, for at least one wavelength within the wavelengthband in use, a chromatic dispersion of −18 ps/nm/km or less and aneffective area of 17 μm² or more.

[0019] Further, when the optical fiber according to the presentinvention is employed as an optical fiber having an enlarged effectivearea, it is preferable for the optical fiber to have, for at least onewavelength within the wavelength band in use, an effective area of 110μm² or more. The optical energy density per unit cross-sectional areacan be kept low in this optical fiber as well, whereby the occurrence ofnonlinear optical phenomena can be suppressed effectively.

[0020] In various kinds of optical fibers mentioned above, the fiberdiameter is 150 μm or more but 200 μm or less. In the case of adispersion-compensating fiber having such characteristics as thosementioned above, however, its fiber diameter is preferably 140 μm ormore but 200 μm or less since its microbend characteristics are likelyto deteriorate in particular.

[0021] When the optical fiber according to the present invention isemployed in an optical cable, it is preferable for the optical fiber tohave, for at least one wavelength within the wavelength band in use, aneffective area of 17 μm² or more and a chromatic dispersion value of −83ps/nm/km or more, and have a fiber diameter of 140 μm or more but 200 μmor less. Such an optical fiber aimed at cabling can be employed as asingle-mode optical fiber such as dispersion-shifted fiber,dispersion-flattened fiber, dispersion-compensating fiber, or the likeas well.

[0022] As explained in the foregoing, in view of various circumstancesapplicable thereto, the optical fiber according to the present inventionis preferably an optical fiber which has a fiber diameter of 140 μm ormore but 200 μm or less, and also has, for at least one wavelengthwithin the wavelength band in use, an effective area of 17 μm² or moreand a chromatic dispersion value of −83 ps/nm/km or more; and, further,preferably is an optical fiber which has a fiber diameter of 140 μm ormore but 200 μm or less, and also has, for at least one wavelengthwithin the wavelength band in use, an effective area of 17 μm² or moreand a chromatic dispersion value of −48 ps/nm/km or more. Also,depending on the kind of optical fiber employed, the fiber diameterthereof is preferably 150 μm or more but 200 μm or less.

[0023] The present invention will be more fully understood from thedetailed description given hereinbelow and the accompanying drawings,which are given by way of illustration only and are not to be consideredas limiting the present invention.

[0024] Further scope of applicability of the present invention willbecome apparent from the detailed description given hereinafter.However, it should be understood that the detailed description andspecific examples, while indicating preferred embodiments of theinvention, are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention will beapparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a chart showing the refractive index profile (doublecore structure) of a dispersion-shifted fiber;

[0026]FIG. 2 is a graph showing the relationship between the mode fielddiameter (Petermann-I) and the increase in loss caused by microbend at awavelength of 1.55 μm in the dispersion-shifted fiber shown in FIG. 1;

[0027]FIG. 3A is a view showing a cross-sectional structure which iscommon in individual embodiments of the optical fiber according to thepresent invention, whereas FIG. 3B is a chart showing the refractiveindex profile of the optical fiber according to the fourth embodiment;

[0028]FIG. 4 is a table listing characteristics of four samples preparedas prototypes for explaining the optical fiber according to the firstembodiment;

[0029]FIG. 5 is a graph showing respective results of evaluation of thefour samples prepared as prototypes for explaining the optical fiberaccording to the first embodiment;

[0030]FIG. 6A is a view showing a cross-sectional structure of anoptical fiber unit constituting a part of an optical cable, whereas FIG.6B is a view showing a cross-sectional structure of the optical cablehaving the optical fiber unit shown in FIG. 6A;

[0031]FIG. 7 is a table listing characteristics of three samplesprepared as prototypes for explaining the second embodiment of theoptical fiber according to the present invention;

[0032]FIG. 8 is a table listing characteristics of four samples preparedas prototypes for explaining the third embodiment of the optical fiberaccording to the present invention;

[0033]FIG. 9 is a graph showing respective results of evaluation of thefour samples prepared as prototypes for explaining the optical fiberaccording to the third embodiment;

[0034]FIG. 10 is a graph showing a relationship between fiber diameterand probability of breaking;

[0035]FIG. 11 is a table listing characteristics of two samples preparedas prototypes for explaining the fourth embodiment of the optical fiberaccording to the present invention;

[0036]FIG. 12 is a chart showing a refractive index profile in the fifthembodiment of the optical fiber according to the present invention;

[0037]FIG. 13 is a chart showing a refractive index profile in the sixthand seventh embodiments of the optical fiber according to the presentinvention; and

[0038]FIG. 14 is a graph showing the relationship between chromaticdispersion and dispersion slope in the optical fiber (Δ⁺=0.9%,Δ⁻=−0.44%) according to the sixth embodiment shown in FIG. 13.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] Individual embodiments of the optical fiber according to thepresent invention will now be explained in detail with reference toFIGS. 1, 3A, 3B, 4, 5, 6A, 6B, and 7 to 14.

[0040] Here, as shown in FIG. 3A, the optical fiber 100 according to thepresent invention comprises a core region 110 extending along apredetermined axis and having an outside diameter a, and a claddingregion 120 provided on the outer periphery of the core region 110 andhaving an outside diameter b (coinciding with the fiber diameter),whereas the core and cladding regions are constituted by at least threelayers of glass regions having respective refractive indices differentfrom each other in embodiments which will be explained in the following.Also, in each of the optical fibers according to the respectiveembodiments, the absolute value of chromatic dispersion at a wavelengthof 1.55 μm (1550 nm) is 5 ps/nm/km or less, the Petermann-I mode fielddiameter is 11 μm or more, and the fiber diameter b is 140 μm or morebut 200 μm or less.

[0041] The optical fibers according to the first to third embodimentshave a refractive index profile of double core structure identical tothat shown in FIG. 1, whereas the optical fiber according to the fourthembodiment has a refractive index profile with an outer ringcore/depressed cladding structure as shown in FIG. 3B.

[0042] The refractive index profile shown in FIG. 1 indicates therefractive index in each part on the line L in FIG. 3A. In the opticalfibers of the first to third embodiments, the core region 110 having theoutside diameter a is constituted by an inner core having a refractiveindex n1, and an outside core provided on the outer periphery of theinner core and having a refractive index n2 (<n1), whereas the claddingregion 120 having the outside diameter b (coinciding with the fiberdiameter) is constituted by a single cladding provided on the outerperiphery of the outer core and having a refractive index n3 (<n2).Thus, each of the optical fibers according to the first to thirdembodiments is an optical fiber which is constituted by three glasslayers (the inner core, outer core, and single cladding) and is insuredits single mode in a wavelength band in use.

[0043] On the other hand, the optical fiber according to the fourthembodiment is an optical fiber having a refractive index profile 500with an outer ring core/depressed cladding structure as shown in FIG.3B, whereas the refractive index profile 500 also indicates therefractive index in each part on the line L in FIG. 3A. In particular,in the refractive index profile 500 parts 510 and 520 indicate a coreregion having an outside diameter a and a cladding region having anoutside diameter b, respectively. In the fourth embodiment, the coreregion is constituted by an inner core having a refractive index n1, anintermediate core provided on the outer periphery of the inner core andhaving a refractive index n2 (<n1), and an outer ring core provided onthe outer periphery of the intermediate core and having a refractiveindex n3 (>n2). On the other hand, the cladding region is constituted byan inner cladding provided on the outer periphery of the outer core andhaving a refractive index n4 (<n3), and an outer cladding provided onthe outer periphery of the inner cladding and having a refractive indexn5 (>n4). Thus, the optical fiber according to the fourth embodiment isan optical fiber which is constituted by five layers of glass (the innercore, intermediate core, outer ring core, inner cladding, and outercladding) and is insured its single mode in a wavelength band in use.

[0044] The optical fibers according to the first to seventh embodimentshaving the refractive index profiles mentioned above will now beexplained successively.

[0045] First Embodiment

[0046] First, for explaining the optical fiber according to the firstembodiment, four kinds of optical fibers (sample 1 a to sample 1 d)having substantially the same Petermann-I mode field diameter MFD1 andrespective values of fiber diameter b different from each other wereprepared as prototypes and evaluated. FIG. 4 is a table listingcharacteristics of each of the four kinds of samples 1 a to 1 d preparedas prototypes for explaining the optical fiber according to the firstembodiment.

[0047] The fiber diameter b is about 125 μm in sample 1 a, about 140 μmin sample 1 b, about 150 μm in sample 1 c, and about 160 μm in sample 1d. Here, each of the Petermann-I mode field diameter MFD1 (11.73 to11.88 μm) given by the above-mentioned expressions (1a) and (1b),effective area (69.7 to 72.1 μm²), chromatic dispersion value (−2.2 to−1.9 ps/nm/km), and cutoff wavelength (1.50 to 1.53 μm) is substantiallythe same among the four kinds of samples 1 a to 1 d.

[0048] Such four kinds of samples 1 a to 1 d having respective values offiber diameter b different from each other are obtained by preparingfour kinds of preforms, which use core members having an identicaldiameter, whose outside diameter ratios between core member and claddingmember differ from each other, and drawing them. Further, the peripheryof each of the four kinds of samples 1 a to 1 d is provided with acoating layer, made of the same material, having an outside diameter of250 μm. The values of the mode field diameter MFD1, effective area, andchromatic dispersion are those measured at a wavelength of 1.55 μm (1550nm).

[0049] For each of these four kinds of samples 1 a to 1 d, microbendloss was measured. In this measurement, each optical fiber having alength of 250 m was wound at a tension of 100 g around a bobbin having abarrel diameter of 280 mm whose surface was wrapped with a JIS #1000sandpaper sheet, and the resulting amount of increase in loss wasdefined as the microbend loss. FIG. 5 is a graph showing the respectiveresults of evaluation of the four kinds of samples prepared asprototypes for explaining the optical fiber according to the firstembodiment. It is seen from this graph that the amount of increase inloss, i.e., microbend loss, becomes smaller as the fiber diameter b isgreater.

[0050] When an optical cable having a cross-sectional structure such asthose shown in FIGS. 6A and 6B is being made, the microbend lossincreases upon cabling. For preventing this phenomenon from occurring,it is necessary that the microbend loss be suppressed to about 1 dB/kmor less. Therefore, as can be seen from FIG. 5, it is necessary for thefiber diameter b to become 140 μm or more when the effective area isabout 70 μm².

[0051] Here, FIG. 6A is a view showing the cross-sectional structure ofan optical fiber unit, whereas FIG. 6B is a view showing thecross-sectional structure of an optical cable in which this opticalfiber unit 200 is employed. In FIG. 6A, optical fibers 100 each having aUV-curable resin 150 coated thereon as a coating layer are integratedwith each other about a tension member 151 with a UV-curable resin 152.Further, the periphery of the UV-curable resin 152 is coated with aUV-curable resin 153, so that the optical fiber unit 200 is obtained. Anoptical cable 300 employing the optical fiber unit 200 is obtained byaccommodating into a copper tube 253 the optical fiber unit 200successively covered with a three-part pipe 250 made of a steel andtension piano wires 252 by way of a water-running prevention compound251, and then successively covering the outer periphery of the coppertube 253 with a low-density polyethylene 254 and a high-densitypolyethylene 255.

[0052] Second Embodiment

[0053] Next, in the second embodiment, the comparison betweencharacteristics of a conventional optical fiber which has already beenverified to be free from the problem caused by cabling andcharacteristics of the optical fiber according to the second embodimentwill be explained. FIG. 7 is a table listing characteristics of each ofthree kinds of samples 2 a to 2 c prepared as prototypes for explainingthe optical fiber according to the second embodiment. Among them, sample2 a is a conventional optical fiber which has already been verified tobe free from the problem caused by cabling.

[0054] The fiber diameter b is about 125 μm in samples 2 a and 2 b, andabout 140 μm in sample 2 c. The Petermann-I mode field diameter MFD1given by the above-mentioned expressions (1a) and (1b) is about 10 μm insample 2 a, and about 11 μm in samples 2 b and 2 c. The effective areais about 55 μm² in sample 2 a, and about 65 μm² in samples 2 b and 2 c.Here, each of the chromatic dispersion value (−2.2 to −2.0 ps/nm/km) andcutoff wavelength (1.51 to 1.53 μm) is substantially the same valueamong the three kinds of samples 2 a to 2 c. The periphery of each ofthe three samples 2 a to 2 c is provided with a coating layer, made ofthe same material, having an outside diameter of 250 μm. The values ofthe mode field diameter MFD1, effective area, and chromatic dispersionare those measured at a wavelength of 1.55 μm.

[0055] Using these three kinds of samples 2 a to 2 c, the optical fiberunit 200 having the cross-sectional structure shown in FIGS. 6A and 6Bwas made. A hydraulic pressure of 100 atmospheres was applied to thusobtained optical fiber unit 200, so as to simulate the external pressureapplied to the optical fiber unit 200 upon cabling. As a result, theamount of increase in loss, i.e., microbend loss, in sample 2 b having afiber diameter b of about 125 μm and an effective area of about 65 μm²was 20 mdB/km. On the other hand, the amount of increase in loss, i.e.,microbend loss, in sample 2 c having a fiber diameter b of about 140 μmand an effective area of about 65 μm² was not greater than 0.5 mdB/km,which was the limit of measurement, whereby this sample yielded acharacteristic equivalent to that of the conventional sample 2 a whichhad already been verified to be free from the problem caused by cabling.

[0056] Third Embodiment

[0057] The third embodiment will now be explained. In the thirdembodiment, for optical fibers having a further greater effective area,the effect of reducing microbend loss resulting from the increase infiber diameter was verified. FIG. 8 is a table listing characteristicsof each of four kinds of samples 3 a to 3 d prepared as prototypes forexplaining the optical fiber according to the third embodiment.

[0058] The fiber diameter b is about 150 μm in samples 3 a and 3 c, andabout 170 μm in samples 3 b and 3 d. The Petermann-I mode field diameterMFD1 given by the above-mentioned expressions (1a) and (1b) is about13.2 μm in samples 3 a and 3 b, and about 14.2 μm in samples 3 c and 3d. The effective area is about 80 μm² in samples 3 a and 3 b, and about90 μm² in samples 3 c and 3 d. On the other hand, each of the chromaticdispersion value (−2.0 to −2.2 ps/nm/km) and cutoff wavelength (1.50 to1.53 μm) is substantially the same value among the four kinds of samples3 a to 3 d. The periphery of each of the four samples 3 a to 3 d isprovided with a coating layer, made of the same material, having anoutside diameter of 250 μm. The values of the mode field diameter MFD1,effective area, and chromatic dispersion are those measured at awavelength of 1.55 μm.

[0059] For each of these four kinds of samples 3 a to 3 d, microbendloss was measured. The method of measuring the microbend loss wassimilar to that in the case of the first embodiment. FIG. 9 is a graphshowing respective results of evaluation of the four kinds of samplesprepared as prototypes for explaining the optical fiber according to thethird embodiment. As can be seen from this graph, when the effectivearea is about 80 μm², the microbend loss can be suppressed to its targetvalue of about 1 dB/km or less if the fiber diameter b is about 150 μmor more. On the other hand, when the effective area is about 90 μm², themicrobend loss can be suppressed to its target value of 1 dB/km or lessif the fiber diameter b is about 170 μm or more.

[0060] From the foregoing, it is seen that, even when the effective areais large, the microbend loss can be suppressed to its target value ofabout 1 dB/km or less if the fiber diameter is large. This fact can beexplained by the following as well. Namely, the microbend loss of anoptical fiber is generated when an external force applied to the opticalfiber causes random minute curvatures in the longitudinal direction inthe core region of the optical fiber. This microbend loss isproportional to the square mean of the reciprocals of the radii ofcurvature of the minute curvatures. On the other hand, if the externalforce applied to the optical fiber is constant, then the minutecurvatures occurring in the optical fiber can be suppressed byincreasing the rigidity of the optical fiber. Letting b be the fiberdiameter of the optical fiber, the rigidity (bending moment) I of theoptical fiber is given by the following expression (3):

I=π·b ⁴/64  (3)

[0061] Therefore, increasing the fiber diameter b of the optical fibergreatly improves the rigidity I of the optical fiber, therebysuppressing minute curvatures and greatly reducing the microbend loss.

[0062] For example, when an optical fiber having a fiber diameter of200.3 μm, an effective area of 90.4 μm², a chromatic dispersion value of−2.1 ps/nm/km, a cutoff wavelength of 1.73 μm, and a coating layeroutside diameter of 250 μm was prepared as a prototype, and itsmicrobend loss was measured in a manner similar to that of the firstembodiment, the resulting value was 0.3 dB/km. This characteristic is ona par with that of the conventional optical fiber which has already beenverified to be free from the problem caused by cabling.

[0063] As the fiber diameter b increases, however, the stress in thefiber surface (the surface of the outermost cladding layer) occurringwhen the optical fiber is bent becomes greater than that in theconventional optical fiber, thereby increasing the probability ofbreaking upon bending. Hence, the range of fiber diameter b within whichthe probability of breaking is practically unproblematic will now becalculated by way of trial. The probability of breaking F of the opticalfiber after passing a screening test is given by the followingexpression (4): $\begin{matrix}{F = {1 - {\exp \left( {{- L} \cdot {Np} \cdot \left\{ {\left\lbrack {1 + {\left( \frac{\sigma_{s}}{\sigma_{p}} \right)^{n} \cdot \frac{ts}{tp}}} \right\rbrack^{m/{({n + 1})}} - 1} \right\}} \right)}}} & (4)\end{matrix}$

[0064] where L is the optical fiber length to which a stress is appliedin the state of its actual use, Np is the number of breaks per unitlength at the time of screening test, σs is the stress upon its actualuse, σp is the stress at the time of screening, ts is the time of actualuse, tp is the screening time, n is the fatigue coefficient, and m is aparameter representing the gradient of a Weibull plot.

[0065] It is assumed that smaller-diameter bending in an optical fiberat the time of actual use occurs in a surplus length portion which isleft for fusion connection in a repeater, and that one turn of bendingwith a diameter of 30 mm exists in one repeater at worst. Also, it isassumed that the total optical fiber length of the optical transmissionsystem is 9000 km, in which repeaters are disposed at intervals of 50 kmon average. Then, the fiber length L to which the bending with adiameter of 30 mm is applied becomes 16.9 m in the optical transmissionsystem as a whole. It is also assumed that the number of breaks per unitlength at the time of screening test Np is 2×10⁻⁵, and the stress σp atthe time of screening is 2.2%. Let the time of actual use ts be 25years, and the screening time tp be 1 second. Let the fatiguecoefficient n be 20, and the parameter m representing the gradient ofthe Weibull plot be 10.

[0066]FIG. 10 is a graph showing the relationship between the fiberdiameter band the probability of breaking according to above-mentionedexpression (4) on the foregoing assumption. As can be seen from thisgraph, the probability of breaking becomes higher as the fiber diameteris greater. If the fiber diameter is 200 μm or less, however, then theprobability of breaking is 10⁻⁵ or lower, whereby there is no problem inpractice.

[0067] Fourth Embodiment

[0068] The fourth embodiment will now be explained. Prepared for theevaluation of the fourth embodiment were two kinds of samples 4 a and 4b, each having a refractive index profile of a dispersion-shiftedoptical fiber with the outer ring core/depressed cladding structureshown in FIG. 3B, whose values of Petermann-I mode field diameter MFD1were substantially identical to each other, whereas their values offiber diameter b were different from each other.

[0069] Here, as mentioned above, the refractive index n1 of the innercore, the refractive index n2 of the intermediate core, the refractiveindex n3 of the outer ring core, the refractive index n4 of the innercladding, and the refractive index n5 of the outer cladding haverelationships of n1>n2, n3>n2, and n5>n4.

[0070]FIG. 11 is a table listing characteristics of each of the twokinds of samples prepared as prototypes for explaining the optical fiberaccording to the fourth embodiment. The fiber diameter b is about 125 μmin sample 4 a, and about 150 μm in sample 4 b. On the other hand, eachof the Petermann-I mode field diameter MFD1 (11.98 and 12.17 μm) givenby the above-mentioned expressions (1a) and (1b), effective area (69.7and 72.1 μm²), chromatic dispersion value (−2.1 and −2.2 ps/nm/km), andcutoff wavelength (1.53 and 1.51 μm) is substantially the same valuebetween the two kinds of samples 4 a and 4 b.

[0071] The two kinds of samples 4 a and 4 b having respective values offiber diameter b different from each other as such were obtained bypreparing two kinds of preforms, which used core members having anidentical diameter, whose outside diameter ratios between core memberand cladding member differed from each other, and drawing them. Further,the periphery of each of the two kinds of samples 4 a and 4 b isprovided with a coating layer, made of the same material, having anoutside diameter of 250 μm. The values of the mode field diameter MFD1,effective area, and chromatic dispersion are those measured at awavelength of 1.55 μm.

[0072] For each of these two kinds of samples 4 a and 4 b, microbendloss was measured by a method similar to that in the case of the firstembodiment. As a result, while the microbend loss of sample 4 a having afiber diameter b of about 125 μ was 4.12 dB/km, the microbend loss ofsample 4 b having a fiber diameter b of about 150 μ was 0.74 dB/km,whereby the latter was able to achieve the target value of microbendloss of about 1 dB/km or less, by which no increase in loss would begenerated by cabling.

[0073] Fifth Embodiment

[0074]FIG. 12 is a chart showing the refractive index profile in thefifth embodiment of the optical fiber according to the presentinvention. The optical fiber according to the fifth embodiment is adispersion-flattened fiber, in which the core region having an outsidediameter a is constituted by an inner core having a refractive index n1and an outside diameter of 3.75 μm, and an outside core provided on theouter periphery of the inner core and having a refractive index n2 (>n1)and an outside diameter of 8.25 μm. On the other hand, the claddingregion having an outside diameter b has a depressed cladding structureand is constituted by an inner cladding which is provided on the outerperiphery of the outer core and which has a refractive index n3 (=n1)and an outside diameter of 15.0 μm, and an outside cladding provided onthe outer periphery of the inner cladding and having a refractive indexn4 (>n3, <n2) and an outside diameter b.

[0075] The refractive index profile 600 shown in FIG. 12 indicates therefractive index of each part on the line L in FIG. 3A, in which parts610 and 620 show respective refractive indices in the core region 110and cladding region 120. Further, in the fifth embodiment, the relativerefractive index difference Δ⁺ of the outer core (refractive index n2)with respect to the outer cladding (refractive index n4) is +0.63%,whereas the relative refractive index difference Δ⁻ of each of the innercore (refractive index n1) and the inner cladding (refractive index n3)with respect to the outer cladding (refractive index n4) is −0.60%, eachgiven by the following expression (5):

Δ=(n _(core) ² −n _(cld) ²)/n _(cld) ²  (5)

[0076] In the above-mentioned expression (5), n_(core) is the refractiveindex of the subject glass region, n_(cld) is the refractive index ofthe outer cladding taken as the reference. In expression (5), therefractive indices of the individual glass regions can be placed in anyorder, so that the relative refractive index difference of a regionhaving a refractive index higher than that of the outer cladding becomesa positive value and is represented by Δ⁺, whereas the relativerefractive index difference of a region having a refractive index lowerthan that of the outer cladding becomes a negative value and isrepresented by Δ⁻. In this specification, the relative refractive indexdifference is expressed in terms of percentage.

[0077] The samples prepared for evaluating the fifth embodiment consistof two kinds having fiber diameters of 125 μm and 160 μm, respectively.Also, the periphery of each of these samples is provided with a coatinglayer, made of the same material, having an outside diameter of 250 μm.Though their fiber diameters are different from each other, both of thesamples have a chromatic dispersion value of 0.12 ps/nm/km at 1.55 μm,an effective area of 72 μm² at a wavelength of 1.55 μm, and a cutoffwavelength of 1.187 μm. Also, their dispersion slope is 0.0096 ps/nm²/kmat a wavelength of 1530 nm, 0.0120 ps/nm²/km at a wavelength of 1550 nm,and 0.0265 ps/nm²/km at a wavelength of 1560 nm. Here, the dispersionslope refers to the gradient of the graph indicating the chromaticdispersion value in a predetermined wavelength band.

[0078] The microbend loss at a wavelength of 1.55 μm (1550 nm) wasevaluated in each sample and, as a result, was found to be 1.1 dB/km inthe sample with a fiber diameter of 125 μm and 0.1 dB/km in the samplewith a fiber diameter of 160 μm, whereby it was verified that theincrease in loss caused by cabling was fully suppressed in the latter.

[0079] Sixth Embodiment

[0080]FIG. 13 is a chart showing the refractive index profile in thesixth embodiment of the optical fiber according to the presentinvention. The optical fiber according to the sixth embodiment is adispersion-compensating fiber in which the core region having an outsidediameter a is constituted by a single core having a refractive index n1and an outside diameter a1. On the other hand, the cladding regionhaving an outside diameter b has a depressed cladding structure and isconstituted by an inner cladding which is provided on the outerperiphery of the core and which has a refractive index n2 (<n1) and anoutside diameter b1, and an outer cladding provided on the outerperiphery of the inner cladding and having a refractive index n3 (>n2,<n1) and the outside diameter b.

[0081] The refractive index profile 700 shown in FIG. 13 indicates therefractive index of each part on the line L in FIG. 3A, in which parts710 and 720 show respective refractive indices in the core region 110and cladding region 120. Further, in the sixth embodiment, the relativerefractive index difference Δ⁺ of the core (refractive index n1) withrespect to the outer cladding (refractive index n3) and the relativerefractive index difference Δ⁻ of the inner cladding (refractive indexn2) with respect to the outer cladding (refractive index n3) are eachgiven by the above-mentioned expression (5).

[0082] The samples prepared for evaluating the sixth embodiment arethose, among samples with Δ⁺=+0.9% and Δ⁻=−0.44% as shown in FIG. 14,having characteristics indicated by point P in FIG. 14 in which thechromatic dispersion value at 1.55 μm (1550 nm) is −33 ps/nm/km, thedispersion slope at 1.55 μm (1550 nm) is −0.10 ps/nm²/km, the effectivearea A_(eff) is 28 μm², and the ratio Ra (=a1/b1) of the outsidediameter a1 of the core region to the outside diameter b1 of the innercladding is 0.6. They consist of two kinds having fiber diameters of 125μm and 160 μm, respectively. Also, the periphery of each of thesesamples is provided with a coating layer, made of the same material,having an outside diameter of 250 μm.

[0083] The microbend loss at a wavelength of 1.55 μm (1550 nm) wasevaluated in each sample and, as a result, was found to be 2.3 dB/km inthe sample with a fiber diameter of 125 μm and 0.3 dB/km in the samplewith a fiber diameter of 160 μm, whereby it was verified that theincrease in loss caused by cabling was fully suppressed in the latter.

[0084] Seventh Embodiment

[0085] Further, in the seventh embodiment, optical fibers having afurther enlarged effective area were evaluated. While the preparedsamples have a refractive index profile identical to that of FIG. 13,their effective area is 122 μm² (110 μm² or more). The prepared samplesconsist of two kinds having fiber diameters of 125 μm and 160 μm,respectively. Also, the periphery of each of these samples is providedwith a coating layer, made of the same material, having an outsidediameter of 250 μm. In both of the samples, though their fiber diametersare different from each other, the relative refractive index differenceΔ⁺ of the core (refractive index n1) with respect to the outer cladding(refractive index n3) is +0.28%, the relative refractive indexdifference Δ⁻ of the inner cladding (refractive index n2) with respectto the outer cladding (refractive index n3) is −0.14%, the cutoffwavelength is 1.49 μm, the effective area at a wavelength of 1.55 μm is122 μm², the chromatic dispersion value at a wavelength of 1.55 μm is22.1 ps/nm/km, and the dispersion slope at a wavelength of 1.55 μm is0.062 ps/nm²/km.

[0086] The microbend loss at a wavelength of 1.55 μm (1550 nm) wasevaluated in each sample and, as a result, was found to be 1.3 dB/km inthe sample with a fiber diameter of 125 μm and 0.2 dB/km in the samplewith a fiber diameter of 160 μm, whereby it was verified that theincrease in loss caused by cabling was fully suppressed in the latter.

[0087] As explained in the foregoing, since the optical fiber accordingto the present invention has a fiber diameter of 140 μm or more but 200μm or less, the increase in microbend loss is effectively suppressed,and the probability of breaking caused by bending stresses can belowered to such an extent that it is practically unproblematic. Also, ifthe absolute value of chromatic dispersion at a wavelength of 1.55 μm is5 ps/nm/km or less, and the Petermann-I mode field diameter is 11 μm ormore, then the occurrence of nonlinear optical phenomena is suppressed,whereby the optical fiber can suitably be used as an opticaltransmission line in WDM transmission systems utilizing the wavelengthband of 1.55 μm.

[0088] Without being restricted to the above-mentioned individualembodiments, the present invention can be modified in various manners.For example, the refractive index profile may have any structure withoutbeing limited to the double core structure and segmented core structure.It can also be realized by a ring core type refractive index profile inwhich a ring core region of a ring shape having a higher refractiveindex is provided around a center region having a lower refractiveindex.

[0089] In the present invention, as explained in the foregoing, sincethe fiber diameter is 140 μm or more, the optical fiber has a highrigidity, so that the increase in microbend loss is suppressed, whereasthe probability of the optical fiber breaking due to bending stresses ispractically unproblematic since the fiber diameter is 200 μm or less.Also, if the absolute value of chromatic dispersion at a wavelength of1.55 μm is 5 ps/nm/km or less, then the optical fiber is suitable forWDM transmissions in which this wavelength band is a wavelength in use.Further, in accordance with the present invention, since the mode fielddiameter is 11 μm or more, the optical energy density per unitcross-sectional area is so low that the occurrence of nonlinear opticalphenomena can be suppressed effectively. Therefore, the optical fiberaccording to the present invention is suitable as an opticaltransmission line in WDM optical transmission systems.

[0090] From the invention thus described, it will be obvious that theembodiments of the invention may be varied in many ways. Such variationsare not to be regarded as a departure from the spirit and scope of theinvention, and all such modifications as would be obvious to one skilledin the art are intended for inclusion within the scope of the followingclaims.

What is claimed is:
 1. An optical fiber comprising a core regionextending along a predetermined axis and a cladding region provided onthe outer periphery of said core region, said core and cladding regionsbeing constituted by at least three layers of glass regions havingrespective refractive indices different from each other; said opticalfiber substantially insured its single mode with respect to light at awavelength in use; said optical fiber having, for at least onewavelength in the wavelength band in use, a chromatic dispersion of 5ps/nm/km or less in terms of absolute value and a Petermann-I mode fielddiameter of 11 μm or more; and said optical fiber having a fiberdiameter of 140 μm or more but 200 μm or less.
 2. An optical cablecomprising an optical fiber according to claim
 1. 3. An optical fiberhaving, for at least one wavelength in a wavelength band in use, adispersion slope of 0.02 ps/nm²/km or less in terms of absolute valueand an effective area of 50 μm² or more; said optical fiber having afiber diameter of 140 μm or more but 200 μm or less.
 4. An optical fiberaccording to claim 3, wherein said optical fiber has, at a wavelength of1550 nm, a chromatic dispersion of 5 ps/nm/km or less in terms ofabsolute value and a Petermann-I mode field diameter of 11 μm or more.5. An optical cable comprising an optical fiber according to claim
 3. 6.An optical fiber having, for at least one wavelength in a wavelengthband in use, a chromatic dispersion of −83 ps/nm/km or more but −18ps/nm/km or less and an effective area of 17 μm² or more; said opticalfiber having a fiber diameter of 140 μm or more but 200 μm or less. 7.An optical fiber according to claim 6, wherein said optical fibercomprises a core region extending along a predetermined axis and acladding region provided on the outer periphery of said core region,said core and cladding regions being constituted by at least threelayers of glass regions having respective refractive indices differentfrom each other, and wherein said optical fiber is substantially insuredits single mode with respect to light in the wavelength band in use. 8.An optical cable comprising an optical fiber according to claim
 6. 9. Anoptical fiber having, for at least one wavelength in a wavelength bandin use, a positive chromatic dispersion and an effective area of 110 μm²or more; said optical fiber having a fiber diameter of 140 μm or morebut 200 μm or less.
 10. An optical fiber according to claim 9, whereinsaid optical fiber has, at a wavelength of 1550 nm, a chromaticdispersion of 5 ps/nm/km or less in terms of absolute value and aPetermann-I mode field diameter of 11 μm or more.
 11. An optical cablecomprising an optical fiber according to claim 9.